Classification and nature of hydrogen bonds to betaine. X-ray, 13C CP MAS and IR description of low barrier hydrogen bonds

Abstract

Twenty two crystalline complexes of betaine [(CH3)3N+–CH2–COO−] with different acids are considered. The resulting hydrogen bonds to the betaine oxygen atoms are characterised by X-ray, 13C NMR and IR data. On this basis five categories of the betaine–acid complexes and their hydrogen bonds are described: (1) MO — the molecular complexes with the bonding protons do not transferre to the betaine; (2) IP — the hydrogen bonded ion pairs with the bonding proton at the betaine oxygen atom, (3) BHB — the homoconjugated betaine complexes with the bonding proton at the central region of the hydrogen bonds, (4) PPD — the MO, IP and BHB complexes with the bonding proton position disorder and (5) LBHB — the PPD complexes with low barrier hydrogen bonds. The results indicate conditions and possible inversion mechanisms of the bonding proton positions. For the betaine phosphoric acid complex the proton disorder, inside one of its hydrogen bonds, was confirmed.